An input voltage is supplied to the high-side switching transistor from an input terminal. A power supply voltage is supplied to the low-side switching transistor from a low-potential power supply. The gate controller turns on and off gates of the two switching transistors. An inductor current according to states (an on-state in which the gate is turned on and an off-state in which the gate is turned off) of the gates of the two switching transistors is passed through an inductor. Therefore, an input voltage is converted into an output voltage.
A reverse current is passed through the inductor in an opposite direction to the inductor current under a predetermined condition. The reverse current is passed toward a low-side switching transistor side in the case of a step-down DC-DC converter, and the reverse current is passed toward a high-side switching transistor side in the case of a step-up DC-DC converter. There is a case that a current-direction detecting circuit for detecting the reverse current is provided in order to reduce a power loss caused by the reverse current and in order to improve conversion efficiency degradation caused by the reverse current. The current-direction detecting circuit detects a direction of a current passed through the switching transistor and outputs a detection result to the gate controller. When the reverse current is detected, the gate controller turns off the switching transistor through which the reverse current is passed.